A structure comprising a semiconductor layer (S) on which a thin insulating film (I) and metal control electrodes (M) are formed is known as a MIS structure. A transistor having such a structure to control the electrical current flowing through the semiconductor layer is referred to as a MIS transistor. Where the insulating film is made of silicon oxide, the transistor is called a MOS transistor.
In the past, the activation step (for removing crystal defects produced on impurity implantation) conducted after the implantation of impurities into such MIS transistors has been carried out by thermal annealing. For this step, a high temperature as high as more than 1000° C. is needed. In recent years, there is a demand for lower-temperature processes. Accordingly, alternatives to such high-temperature thermal annealing have been discussed. One promising method is to illuminate laser light or other intense light, for effecting activation. Depending on the used light source, this method is called laser annealing or lamp annealing.
A conventional method of fabricating MIS transistors, using laser annealing, is now described by referring to FIGS. 4(A)-4(E). An insulating film 402 is deposited as a base layer on a substrate 401. Then, a substantially intrinsic crystalline semiconductor film is deposited. This is photolithographically patterned into island-shaped semiconductor regions 403. Thereafter, an insulating film 404 acting as a gate-insulating film is deposited. Subsequently, gate electrodes 405 are deposited (FIG. 4(A)).
If necessary, the gate electrodes are anodized to form an anodic oxide 406 on the top and side surfaces of the gate electrodes and conductive interconnects. This method for forming such an anodic oxide and its merits are described in detail in Japanese Patent application Ser. Nos. 30220/1992, 34194/1992, 38637/1992, etc. Of course, this anodization step may be omitted if not necessary (FIG. 4(B)).
Then, an impurity is implanted by ion implantation or ion (plasma) doping. In particular, the substrate is placed in a fast stream of ions. Using the gate electrode portions, i.e., the gate electrodes and the surrounding anodic oxide, as a mask, an impurity is implanted into the island-shaped semiconductor regions 403 by a self-aligning process. In this way, doped regions 407 which will act as source and drain are formed (FIG. 4(C)).
Thereafter, intense light such as laser light is illuminated to recover the crystallinity which was deteriorated by the previous impurity implantation step (FIG. 4(D)).
An interlayer insulator 408 is then deposited, and contact holes are formed in it. Source and drain electrodes 409 are formed, thus completing MIS transistors (FIG. 4(E)).
In the method described above, when impurities are implanted, a large amount of impurities is introduced also into the gate-insulating film 404. These impurities themselves act as cores absorbing the laser light. In addition, defects produced by the impurity implantation absorb the laser light strongly. Especially, UV light is absorbed much, and light strong enough to activate the doped semiconductor regions 407 does not reach these regions. Usually, the insulating film is made of silicon oxide. The laser light is emitted from an excimer laser which has an excellent mass-producibility. If the silicon oxide is pure, it is sufficiently transparent to UV light emitted from an excimer laser. However, if impurities such as phosphorus and boron are present, the transparency deteriorates greatly. Hence, the activation is not sufficiently done.
If the doped regions are not sufficiently activated in this way, their resistivities are increased. It substantially follows that a resistor is inserted in series between source and drain. That is, the apparent mobility of the transistor drops. Also, the rising characteristics, or steepness, obtained when the transistor is turned on deteriorate.